(1) Field of the Invention
The invention relates to gas turbine engine components, and more particularly to an airfoil insert for discharging an increased volume of cooling air.
(2) Description of the Related Art
In a gas turbine engine, incoming air is pressurized by a compressor and mixed with fuel in a combustor. The fuel and air mixture is burned and expelled from the combustor as hot combustion gases. The hot combustion gases are directed to a turbine disposed downstream of the combustor, where the turbine extracts power from the gases and rotates the compressor via a common shaft.
The turbine is comprised of alternating axial stages of rotating blades and stationary vanes. The blades within each stage are circumferentially spaced about a disk attached to the common shaft, whereas the vanes are cantilevered inward from an outer casing structure. A spacer located radially inboard of the vanes, controls the axial spacing of successive bladed disks. A rotating seal, affixed to the spacer, discourages interstage leakage of the combustion gases by mating with a stationary land attached to the inner diameter of the vanes. The interstage seal and land are crucial to the operating efficiency and performance of the gas turbine engine.
Protecting turbine components from the hot combustion gases is very important, since the combustion gas temperature may exceed the melting temperature of the component's base material. For protection, these components are typically insulated with high-temperature coatings and convectively cooled with a portion of the compressor air. This portion of the compressor air bypasses the combustion process and is hereinafter referred to as cooling air.
Since the interstage seal and land are located radially inboard of the vanes, the cooling air must first be channeled through the vanes to reach them. Typically, a tubular insert is located inside each vane to apportion the cooling air between the vane and the interstage seal and land. The insert is open at a first end to allow cooling air to enter from an outboard annular plenum, and is perforated along its length to generate impingement-cooling jets within the vane. The second end of the insert is partially restricted by a perforated cover to increase the velocity of the impingement-cooling jets in the vane and to allow for a portion of the cooling air to discharge to the interstage seal and land. The cover also adds structural strength to the tubular insert, which may deform during assembly and from the extreme combustion gas temperatures.
As the cooling air passes through the vanes and other components, its temperature increases, diminishing its ability to cool the interstage seal and land. Since the longevity of the interstage seal and land is crucial to maintaining the overall efficiency and performance of the gas turbine engine, any improvement in durability is advantageous. If the operating temperature of the interstage seal and land is reduced, the durability is improved and the serviceable life is extended. Utilizing a lower temperature cooling air source, or providing a greater volume of available cooling air will reduce the operating temperature of the interstage seal and land. Since a lower temperature cooling air source does not have sufficient pressure to ensure constant flow, then the vane insert must distribute an increased volume of available cooling air to the interstage seal and land.
Reducing the level of restriction in the second end of the insert increases the volume of cooling air; however, simply adding additional perforations in the existing cover will weaken the cover and make it more susceptible to thermal fatigue cracks and oxidation. Introducing oblong holes in the existing cover is expensive and the remaining cover material is susceptible to cracking and oxidation. Removing the existing cover entirely reduces the velocity of the impingement-cooling jets in the vane and jeopardizes the structural integrity of the insert.
What is needed is an insert for distributing an increased volume of available cooling air to the interstage seal and land, without reducing the velocity of the impingement-cooling jets or diminishing the structural integrity of the insert. Additionally, the insert must be capable of being produced in a robust and repeatable manner, with existing manufacturing processes and tooling and at a reasonable cost.